Dependence of the Radiation Response of an in Vitro Tumor Model on Cell Cycle Effects¹

Multicellular “spheroids” of Chinese hamster V79 cells grown in suspension culture are morphologically similar to areas of some solid tumors. Single cells from spheroids were obtained by trypsinization and assayed for colony-forming ability after intact spheroids of different sizes were exposed to ionizing irradiation. The multicomponent survival curves thus obtained were shown to be the result of cells residing in three states of radiosensitivity: radiosensitive G₁-like noncycling cells, more resistant asynchronously cycling cells, and highly radioresistant hypoxic internal noncycling cells. The influence of intercellular contact as cells progressed through the cell cycle was discussed with regard to radiotherapy.